Oil sands are known to contain hydrocarbons known as bitumen trapped between individual grains of the oil sands. Water surrounding the grains contains very fine clay particles, fine sand, and silt (referred to in the art as “fines”). The bitumen from the oil sands may be recovered by forming a slurry including the oil sands dispersed in an aqueous solution (e.g., water) that may include caustic (e.g., NaOH) in addition to the water. The slurry is fed into a primary separation vessel (PSV) (e.g., a floatation cell) where the oil sands are aerated. Air is bubbled through a bottom portion of the PSV, such as with a sparger, and aerated bitumen rises to the surface to form a froth that overflows the PSV and is recovered for further treatment. Eventually, the recovered bitumen may be upgraded to crude oil, such as by fluid coking, hydro processing, hydro treating, and reblending.
Gravitational forces cause the sand grains to sink and concentrate at the bottom of the PSV. Middlings, a watery mixture containing suspended fines and bitumen, extend between the froth and the sand layers. The underflow and the middlings are frequently combined and processed in a secondary floatation process (known as a Tailings Oil Recovery (TOR) vessel) to recover any bitumen that may remain in the tailings or in the middlings. The middlings and an underflow from the TOR may be further processed to recover any unrecovered bitumen, or may be discarded as tailings. The middlings and the underflow from the TOR vessel may eventually be sent to a tailings pond. In some instances, the underflow contains mainly coarse sands, which may be pumped to a tailings deposition area.
However, the middlings may include coarse sands, mineral fines, and dissolved metals (e.g., mercury, arsenic, etc.) and are, therefore, not suitable for direct discharge into the environment (e.g., rivers). Thus, the middlings are frequently discharged into a tailings pond where the fines are allowed to settle under gravitational forces. Unfortunately, the fines remain stable in the tailings and may take months to several years to settle. This excessive duration presents issues in the recovery of bitumen from oil sands.